This invention relates to underground well devices and processes. More specifically, the invention is concerned with providing a device and method that reduces the risk of uncontrolled formation fluid influx into a well during operations requiring removal of tubular strings from the well.
During the drilling, completion, operation, and maintenance of an underground wellbore, e.g., drilling a well to produce oil or gas from an underground formation or reservoir, frequent round trips of tubulars in and out of the wellbore or xe2x80x9ctrippingxe2x80x9d is required. One round trip, for example, might involve removing sections of a drill string from a wellbore otherwise filled with a drilling mud, replacing a worn-out drill bit at the end of the drill string, and returning the string (with a new bit) to the wellbore to continue drilling. As the drill string and/or attached equipment is being removed or pulled out of the wellbore, a replacement fluid is typically supplied from a trip tank to replace the volume of removed drill string and/or attached equipment so that a steady-state hydrostatic pressure is maintained downhole. The replacement fluid is typically also a drilling mud or similar fluid.
However, the string removal operation itself can induce an unwanted fluid exchange between the wellbore and an underground formation. For example, a formation fluid may be induced into the wellbore from an upward swabbing effect of drill string removal that produces frictional forces to help support the column of drilling mud near the top of the wellbore and reduces hydrostatic pressure near the bottom of the string. If the induced formation fluid displaces the drilling mud in the wellbore and has a density lower than the displaced drilling mud, e.g., if the formation fluid is a gas, the hydrostatic pressure near the bottom of the drill string is reduced still further. The reduced hydrostatic pressure can induce the entry of more formation fluids, causing a surface pressure xe2x80x9ckickxe2x80x9d or other problems.
Similarly, insertion of tubulars can increase bottomhole hydrostatic pressures. Increased bottomhole pressures can force unwanted flows of drilling mud into an underground formation.
In order to detect a kick or other problems during tripping, the volume of liquid supplied from the trip tank over a time period may be measured and compared to a calculated volume of liquid needed to replace the volume of steel drill pipe and other steel equipment removed during the time period. If the calculated steel volume removed is greater than the volume of liquid supplied from the trip tank, then an influx of formation fluid can be assumed to have entered the wellbore and appropriate corrective steps may be taken. If the calculated steel volume is less than the supplied fluid volume observed from the trip tank, drilling or other fluid in the wellbore can be assumed to have entered the underground formation (potentially damaging the formation) and appropriate corrective steps may be taken.
Unfortunately, it is not unusual for the calculated steel volume to be somewhat more or less than the injected fluid volume from the trip tank in the absence of the fluid transfer between the wellbore and a formation. This can be caused by a differential xe2x80x9cswabbingxe2x80x9d effect of the drill pipe or other equipment being moved within the wellbore resulting in an exchange of fluid within the wellbore, i.e., fluid exchange between the interior and annular exterior of the drill string rather than fluid exchange between the wellbore and an underground formation. Because of the differential swabbing effect and/or other possible reasons, some differences between injected volume (from the trip tank) and removed tubular volumes are typically ignored rather than taken as an early sign of unacceptable fluid exchange with a formation since unnecessarily stopping a round trip can be costly.
But perhaps even more costly or dangerous is not being able to detect the early signs of a kick or other fluid exchange problems. For example, controlling an early-detected small kick may be relatively easy but lengthy delays in detection can result in short tripping, kick control measures such as extensively circulating out the (presumed) kick gas, killing the well, actuation of blowout preventers, or even a well blowout. Similarly, early detection of unwanted drilling fluid or circulation loss into the formation again may be easily controlled or only result in xe2x80x9cskin damagexe2x80x9d to the formation, but delays in detection and excessive drilling fluid loss can result in lost circulation or drilling problems, permanent formation damage after drilling, and even loss of the well.
In accordance with the present invention, improved methods and apparatus are described for detecting a fluid exchange between a wellbore and an underground formation. The process of the invention allows one to quickly calculate (using the principle of conservation of mass) and/or correct unwanted fluid exchange between the wellbore and an underground formation by directly measuring essentially all other volume inputs/outputs and volume changes with a wellbore.
One apparatus embodiment of the invention comprises a first acoustic sensor (typically attached to a drill string) for detecting liquid level/volume changes within a drill string, a second or trip tank liquid-level sensor (typically attached to a trip tank) for detecting liquid level/volume changes in a trip tank, and means for measuring the volume of drill string removed from a well, such as counting the number of drill string sections removed over time multiplied by a volume of steel per string section. Another apparatus embodiment of the invention comprises a first liquid volume change or liquid flow sensor attached to a top drive of a drilling rig, a second or trip tank liquid flow sensor attached to the outlet of the trip tank, and means for measuring the distance traveled by drill string tubulars such as a drawworks position indicator. The first liquid level or volume change sensor detects signals reflected from a liquid-level interface within the drill string over time, allowing calculation of measured liquid flow or volume changes within the drill string The trip tank sensor allows calculation of liquid volume exchanges or flow between the trip tank and the wellbore. The drawworks sensor detects movement of the drill string into or out of the wellbore over time. Calculated liquid and solid volume changes within the wellbore over time are compared or totaled. Discrepancies between inflow and outflow or non-zero volume change totals are used to determine if fluid exchange with an underground formation has occurred and/or to take corrective action.
One process embodiment of the invention comprises: (a) drilling a wellbore using a drilling rig and tubulars extending into said wellbore; (b) transmitting a liquid-level interacting signal into said wellbore over time; (c) detecting a plurality of said fluid level-interacting signals over a period of time and calculating an associated measured liquid volume change; (d) measuring the volume change of tubulars withdrawn or inserted into said wellbore over said period of time; (e) measuring a fluid volume change or liquid flow into or out of said wellbore from a source other than said underground formation over said period of time; (f) totaling said measured volume changes; and (g) correcting unwanted formation fluid fluxes if the totaling step indicates fluid has been exchanged between the wellbore and an underground formation.